Field of the Application
This disclosure generally relates to circuit boards, and more specifically to a circuit board used in light emitting device (LED) applications.
Background of the Disclosure
Light emitting devices (LEDs) are becoming a part of everyday life. LEDs are used in widely-varying applications, such as traffic lights, Christmas lights, automotive indicator lights, walk/don't walk and exit signs, flashlights and indoor/outdoor utility lighting. In this last category, indoor/outdoor lighting, LEDs are used in such items as: indoor tubes to replace fluorescent tubes, stage lights, indoor/outdoor display lights, outdoor flood lights and indoor screw-in light bulbs.
Because some LED lights are replacements for already-existing products (like bulbs and tubes), the general form-factor for the LED products is somewhat defined. For example, an LED light bulb that replace a regular, screw-in incandescent bulb should be of about the same shape and size as the regular light bulb and have the same screw-in threads as the regular bulb. Therefore, the inner workings of each type of LED light are mostly constrained to the form-factor of each type of light it replaces. LED light bulb parts like LED diodes or LED arrays, the light diffusers and transmitters, any power supply or other electronics, the circuit board(s) on which the LED(s), LED array(s) and/or other electronics may be mounted, and any heat dissipating sinks all should to fit within the general size and shape of the regular bulb it replaces.
Additionally, because LED light bulbs (or tubes) are replacing already existing bulbs (or tubes), their safety, reliability and durability should be at least equal to the standards and expectations set by the light bulbs (or tubes) they are replacing. Safety tests exist, such as those defined by Underwriters Laboratories, Inc., which are used for certifying both incandescent and LED light bulbs (as well as fluorescent and LED tubes) and include tests like: environmental sealing, mechanical strain relief and electrical strength or dielectric withstand.
In the context of circuit boards for LED lighting applications, electrical strength or dielectric withstand voltage is affected by the electrical creepage distance, which is the shortest path between two conductive parts (or between a conductive part and a conductive mounting piece) of an LED bulb or pieces of the bulb measured along the surface of the insulation or insulator. Proper creepage distance helps protect components and users by ensuring that conductive parts of the LED light, which are not supposed to conduct electricity between them, do not conduct electricity (or electrical arcing) under certain testing and/or operating conditions of the LED bulb or tube. Likewise, proper creepage distance can help prevent a shocking hazard by ensuring that conductive parts of the LED light do not conduct electricity (or electrical arcing) with the mounting structure(s) of the LED bulb under certain testing and/or operating conditions.
For non-isolated, mains powered, replacement LED light bulbs, creepage might be problematic, especially for the small circuit board on which an LED diode or LED array (and/or other electronics) may be mounted. Such a circuit board should be small enough to fit within the dimensions of the bulb or tube it is replacing and yet still include proper creepage distances needed to pass electrical strength certifications when tested for non-isolated, mains powered applications. For example, an isolated, mains powered creepage test may only require sustaining about 500 volts AC without electrical arcing, yet a non-isolated, mains powered creepage test may require sustaining from about 1,240 volts AC (United States testing) to more than 3,700 volts AC (European testing) without arcing between conductive pieces of the circuit board and/or the enclosure in which the circuit board may be mounted (i.e., the enclosure of an LED light bulb).
Therefore what is needed is a circuit board on which an LED diode or LED array (and/or supporting electronics) may be mounted, which fits and functions within the application of a replacement light bulb (or tube), and passes non-isolated, mains powered, electrical creepage testing.